1. Field of the Invention
The present invention relates to heat exchangers and more particularly to a heat exchanger especially adapted for use in a multi-solid, fluidized bed, heat generating system wherein hot fine solid particulate material is recirculated between the heat exchanger and a combustion reactor vessel. The heat exchanger of the present application comprises a modified form of heat exchanger which differs from the external heat exchanger that is disclosed in copending U.S. Pat. application Ser. No. 939,819, filed Dec. 9, 1986 now U.S. Pat. No. 4,709,663 granted Dec. 1, 1987 (Larson et al), which application is assigned to the same assignee as the present application.
2. Description of the Prior Art
Fluidized bed, multi-solid, heat generating systems such as those disclosed in U.S. Pat. Nos. 4,084,544 and 4,154,581, include a combustion vessel having a dense bed section containing relatively large size or coarse solid particulate material maintained in a fluidized state in a limited area within the vessel for pulverizing the fuel and retaining at least a portion of the pulverized material in the bed while mixing with oxygen for controlled combustion in and above the dense bed section. Fluidized solid fines or small size solid particulate material is recirculated between the combustion vessel and an external heat exchanger. These fines are adapted to absorb heat generated in the combustion process within the combustion vessel and thereafter give up the heat in the external heat exchanger to generat steam before the fines are recirculated back into and through the dense bed section of the combustion reactor vessel. A relatively high percentage of the heat energy developed in the combustion process is transferred in the external heat exchanger from the recirculating fine solid particulate material to the water/steam moving through heat exchange coils that are provided in the external heat exchanger body or enclosing structure.
The heat exchanger of the present invention is especially designed and adapted to provide a more efficient heat transfer between the recirculating fine solid particulate material and the feed water/steam which is to be heated by the energy developed in the combustion process and carried by the recirculating solid fine particulate material.